Chapter 9: Covalent Bonding and Molecules

Steric number (SN)

(number of atoms bonded to central atom) plus (number of lone pairs on the central atom).

Molecular shape

-a bond angle is the angle defined by lines joining the centers of two atoms to a third atom to which they are covalently bonded -The geometry is defined by the lowest energy arrangement of its atoms in a three-dimensional space. -governs the chemistry behind them

Central atoms with Lone Pairs

-electron-pair geometry describes the arrangement of atoms and lone pairs of electrons around a central atom (it will always be one of the three geometries presented) -molecular geometry in these molecules describes the shape of the atoms present (it excludes the lone pairs). -plays and important role in why chemical properties are what they are -lone pairs affect bond angles (actually take up space)

Geometric forms in electronic geometry

-linear -trigonal planar -tetrahedron

Two general types of covalent bonds

-polar covalent and nonpolar covalent Polar bonds occurs when atoms are bonded fo F, Cl, or O

Lewis Structures

-shows how valence electrons are arranged among atoms in a molecule -reflects central idea that stability of a compound relates to noble gas electron configuration (octet rule)

Exceptions to the octet rule.

-some molecules have fewer than eight electrons in a lewis structure (BF₃ for example). -these are known as electron deficient compounds. -other molecules have more than an octet (SF₄ for example).

Electronegativity and bond polarity

-the electrons in a covalent bond are not always shared equally by the two bonded atoms -unless the bond connects two atoms of the same element, there will always be one atom that attracts the electrons in the bond more strongly than the other atom does. -when an imbalance occurs, there is a resulting build of some negative charge, called a partial negative charge, on one side of the bond and some positive charge on the other side.

Predicting a VSEPR Structure

1) Draw the Lewis structure. 2) Determine the steric number of the central atom. 3) Use the SN to determine the geometry around the central atom. 4) The name for the molecular structure is determined by the number of lone pairs and bonding pairs of electrons.

Steps to make electron-dot structures of polyatomic molecules:

1) count the valence electrons for all atoms and determine the central atom (listed first and exceptions will be stated) 2) Make the bonds between the atoms 3) Assign the remaining electrons after bonding to terminal atoms to make octets. 4) if unassigned electrons remain after 3), assign them to the central atom 5) If the central atom does not have an octet - and no more electrons remain, use one or more lone pairs from a terminal atom to form a multiple bond.

Molecular geometry

A description of the arrangement of atoms within a molecules; the molecule shape.

Electronic geometry

A description of the arrangement of electrons around a central atom. -number of electron clouds around a central atom

Tetrahedral

A geometry in which four atoms or electron sets are separated by 109.5° angles.

Trigonal planar

A geometry in which three atoms or electron sets are separated by 120° angles.

Linear

A geometry in which two atoms or electron sets are separated by a 180° angles.

Electronegativity

A measure of how strongly atoms pull bonded electrons. A relative measure of how strongly an atom attracts electrons when it forms a covalent bond. To judge the relative polarity of a covalent bond, chemists use the difference in ________ between bonded atoms F is the most negative in the room.

Lone pair

A pair of unshared valence electrons.

Molecular dipole Net dipole Dipole

An overall polarity in which different sides of a molecule have slight positive and negative charges.

What is the octet rule?

Atoms are stabilized by completely filled valence shells (per this course, no s and p sub levels covered). This corresponds to 8 electrons in the valence shell (except H which is fulfilled with two).

Physical properties and Polarity

Because water is a polar molecule (negative side and positive side), each molecule has an attraction for the other that carbon dioxide does not. This produces intermolecular forces. This is what contributes to water's comparatively high freezing points and boiling points compared to methane and ammonia.

Polar covalent bond

Bond in which the atoms do not share the electrons evenly; in this type, one atom has a slight positive charge, while the other has a slight negative charge.

6 bonded electrons and 6 non bonded electrons are represented.

How many bonded electrons are resented in this lewis structure? How many non bonded electrons are represented?

What is the electronic geometry and bond angle that would occur around an atom with 2 charge sets?

Linear, 180°

How do nonmetals fulfill the octet rule?

Nonmetals share valence electrons.

What is the electronic geometry and bond angle that would occur around an atom with 4 charge sets?

Tetrahedron, 109.5°

Number of valence electrons of CH₃Cl

There are 14 total valence electrons.

Number of valence electrons in HNO₃

There are 24 total valence electrons.

Number of valence electrons in PCl₃

There are 26 total valence electrons.

Number of valence electrons in H₂SO₄

There are 32 valence electrons in sulfuric acid, 16 in bonds and 8 lone pairs.

Number of valence electrons in NH₃

There are eight total valence electrons, 6 in bonds and one lone pair.

Number of valence electrons in HCl

There are eight total valence electrons, two in a bond, 3 lone pairs

What is the electronic geometry and bond angle that would occur around an atom with 3 charge sets?

Trigonal planar, 120°

Valence shell electron pair repulsion theory

VSEPR model A way of predicting the geometry of molecules based on the number of electron sets around a central atom.

How many total number of valence electrons in SO₃²⁻

Without a charge, it would be 24, but with a negative 2 charge, it is 26 total valence electrons.

How many total number of valence electrons in BH₄⁻

Without a charge, it would be 7, but with a negative charge, it has 8 total valence electrons.

How many total valence electrons in NH₄⁺

Without a charge, it would be nine, but with a positive charge, it is 8 total valence electrons.